The Range of Environmental Concerns - Mining and the Environment: From Ore to Metal - page 545

rocks above drainage store relatively little methane because coal exposed at an outcrop
loses the methane during the erosional process. Thus, much methane is associated with
deeper coal seams. However, because coal has such a large internal surface area (the micro-
pores in coal provide a large surface area, approximately 200 to 300 square metres per
gram of coal; OSM 2001), it can store surprisingly large volumes of methane-rich gas; six
or seven times as much gas as contained in a typical natural gas reservoir of equal volume.
Actual methane gas quantities are, as might be expected, a function of depth, pressure,
water/moisture content and the extent of coalii cation. Depth is important because it affects
the pressure and temperature of the coal seam, which in turn determines how much meth-
ane is generated during coal formation (Williams and Mitchell 1994). If two coal seams
have the same rank the deeper seam will hold larger amounts of methane because pressure
increases with depth, all other things being equal. Water content is important, as when the
coal and methane conversion process occurs and coal is saturated with water, methane is
more easily trapped within the coal.
The translation of methane content in coal into methane gas emissions during mining
is a complex matter, and so is the calculation of methane gas emissions from coal min-
ing. The rate of methane gas release depends on the quantity and the way coal is mined,
the working depths, mine drainage, the types of ventilation in case of underground
coal mining, and many other factors such as density of coal or the general geology of
the mining area. The OECD Expert Group recommends global average emission fac-
tors for underground coal mining between 10 and 25 M 3 CH 4 /tonne (OECD 1990). The
simplest estimate of annual coal mining methane (CCM) emission is then calculated as
follows:

Because coal has such a large
internal surface area, it can store
surprisingly large volumes of
methane-rich gas; six or seven
times as much gas as contained
in a typical natural gas reservoir
of equal volume.

CCM Emissions (tonnes) Emission Factor Coal Production (tonnes/annum)

Conversion Factor

The conversion factor converts the volume of CH 4 to a weight measure based on the den-
sity of methane.
Little data exists on which to base emission factors from surface mining. Average emis-
sion factors ranging from 0.3 to 2 M 3 CH 4 /ton are found in the literature (USEPA 1999),
but these numbers are more uncertain then the corresponding factors for underground
mining. However, it is clear that underground coal mines are the single largest source of
coal mine methane (CMM) emissions in most countries. Emission factors do not account
for post-mining methane emissions, commonly overlooked in many past studies.
In total, CMM accounts for an estimated 8% of total methane emissions resulting from
human activities. In 2000, worldwide CMM emissions totalled 120 million tonnes of car-
bon equivalent (MMTCE), or about 30.8 billion cubic metres (BCM). By 2020, the world's
coal mines are expected to produce annual emissions of 153 MMTCE (39.3 BCM) (meth-
anetomarkets 2006). China and the United States, the world two largest producers of hard
coal, are also the leading emitters of CMM, contributing about 35% (China) and 25%
(USA) to global CCM emissions. Both countries also have made no commitment or obli-
gation to reduce GHG emission under the Kyoto protocol.

CMM accounts for an estimated
8% of total methane emissions
resulting from human activities.

Coal Bed Methane Capture

Coal mining does not always cause uncontrolled release of methane into the global atmos-
phere. Coal bed methane, previously a wasted energy resource, is being increasingly